Telecommunication system Including a local area network (LAN), an asynchronous transmission mode (ATM) network, and a broadband data service (BDS) network in which a protocol data unit is not reconfigured in the (BDS) network

ABSTRACT

The invention relates to a device for broadband data service transmission in telecommunication system. The invention aims at implementing bridges in public ATM network, by which public data communication services can be used by private, local networks LAN. The bridges make use of a connectionless data service function by means of broadband data service servers (BDS). The ATM network uses the protocol AAL5. According to the invention the transmission can by that be made without buffering. Preferably respective local network is connected via only one BDS server and the transmission in the BDS network is in streaming mode.

This application is a 371 of PCT/SE95/00829 filed Jul. 7, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for broadband data servicetransmission in a telecommunication system. The invention is aiming atimplementation of bridges in public ATM networks (AsynchronousTransmission Mode) by which public data communication services can beused by private, local area networks (LAN). The bridges use aconnectionless data service function by means of broadband data serviceservers BDS (Broadband Data Service). The ATM-network uses the protocolAAL5 (ATM Adaption Layer 5).

2. Discussion of the Background

It is previously known to connect a local network to an ATM network bymeans of converting units outside the ATM network. Packing and depackingof the information occurs in each node. This requires that the serverhas a buffer and can maintain many connections at the same time, whichcauses delays in the system. A number of patent documents are also knownwithin the field.

WO 92/2189 shows an interface for connection of local networks to ISDN(Integrated Services Digital Network). The interface is transparent fora local network user. In order to optimize the use of bandwidth, thepackets are packed together to a train of packets which is compressedbefore it is transmitted.

U.S. Pat. No. 5.229.994 describes a bridge for connection of a localnetwork to an asynchronous telecommunication network. The bridge allows"layer 2.1" for communication with the network.

WO 93/26107 relates to an "ATM-ETHERNET portal/concentrator" whichallows transparent connection of Ethernet segment over an ATM network.By using a double port memory the construction becomes simple, theprocessor power slight and the capacity high, because there is no needto copy data cells.

U.S. Pat. No. 5.179.555 shows a bridge for connecting data trafficbetween local networks and a WAN (Wide Area Network). Data which are tobe transmitted in the WAN is compressed in frames to get hightransmission capacity. After each compressed frame, a marker is placedcalled "end of compression".

EP, A1 0 473 066 relates to a system for connection of local networks toan ATM network. In order to increase the transmission rate for cellsfrom one bridge to another, complete logical connections are, as far aspossible, used.

The above documents shows different ways of using bridges for connectionof traffic between different networks, for instance local networks andan ATM network. The bridges in the shown documents use different ways inorder to, in an efficient way, transfer data packets. None of thedocuments describe bridges which do not buffer traffic and which makesuse of AAL5.

SUMMARY OF THE INVENTION

According to the invention the traffic is transmitted all the way bycells in order to avoid packing and unpacking at each node. This leadsto better performance, simpler nodes, and escaping the need to buffertraffic. Consequently, the need for a large memory capacity is avoided.

Accordingly, the present invention provides a device for broadband dataservice transmission in a telecommunication system, comprising localnetworks connected to ATM network via a broadband data service network.According to the invention, the transmission does not need buffering.

Preferably the transmission occurs in the BDS network in Streaming Modeand a connectionless service function is established by means of BDSservers.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail with reference to theenclosed drawings, in which:

FIG. 1 shows a diagram over the network structure according to theinvention,

FIG. 2 presents the protocol AAL374 compared with AAL5,

FIG. 3 shows the routing in the BDS server, and

FIG. 4 presents an example of software modules for BDS according to theinvention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The present invention relates to a device for implementing bridges inpublic ATM networks (Asynchronous Transmission Mode) by which publicdata communication services can be added to the system in a transmissionefficient way. It is often desirable to be able to transmit tele- anddata traffic from, for instance, one local network to another localnetwork via a telecommunication system. One of the most importantapplications concerning broadband ISDN will be local network connectionsover the ATM network. FIG. 1 shows the structure of the differentnetworks and connections.

The BDS network can be of two different configuration types: theindirect method and the direct method. The indirect method requires thatthe BDS network is installed outside the ATM network. It can be regardedas a network with leased lines where a more or less fixed bandwidth isallocated to the logical channel between between local network routersor servers in question.

The present invention, however, intends to use the direct method. Thedirect method implements a connectionless service function by means oftwo BDS servers. These work within the ATM network and are connectedeither permanently or semi-permanently. Each server performs routing bydeciding the next jump to which a cell must be transmitted in order toreach its final destination, such as a router or broadband terminal.With the direct method, local networks only require one single ATMconnection to a BDS server. If the connections are permanent orsemi-permanent between the BDS servers, the total number of connectionsrequired in the BDS network by the end users is essentially reduced.

In the ATM network, two adaption layer protocols are relevant: Theprotocol AAL3/4 and AAL5. The difference between the two protocols isshown in FIG. 2.; AAL3/4 and AAL5 differ regarding packing, unpackingand repacking of the user data to/from ATM cell pay load (orSegmentation and Reassembly Protocol Data Units, SAR PDU) andmultiplexing. In the figure, payloads are white, whereas headers and endparts are broken lines.

In AAL3/4, the CPCS protocol data unit (Common Part ConvergenceSublayer) receives user data and adds its own header and end part tothis. In Message Mode, the whole user block is received before anythingis forwarded to the ATM layer. In Streaming Mode, the user data blockcan be treated in pieces. After a procedure where the CPCS protocol dataunit is segmented in pieces of 44 octets, these octets are combined witha header of two octets and an end part of two octets for finishing theAAL3/4 SAR protocol data unit. Notice that the header of the SARprotocol data unit contains a MID-field (Multiplexing ID), which allowsthe transmitter and the receiver to multiplex different user datasources and sinks over the same ATM connection; and that the SARprotocol data unit end part contains a 10-bits checking sum code. Thischecking sum is calculated over the whole SAR protocol data unit.

AAL5, on the contrary, only adds an end part to the user data. The endpart contains among other things a checking sum field, which iscalculated over the whole CPCS protocol data unit (CRC32). The divisionof this protocol data unit into ATM cell payloads (AAL5 SAR PDU) is,however, more straightforward than for AAL3/4, because the payload ofthe whole cell is used for the CPCS protocol data unit. No further checksum is supplied in the ATM-cell beside the one which is associated tothe header of the ATM-cell.

Using AAL5 instead of AAL3/4 as the protocol on which the performance ofthe BDS network is based leads to several advantages:

The MID-field att AAL3/4 SAR protocol data unit is no longer regarded asnecessary due to the large number of logical channels VPI/VCI (VirtualPath identity/Virtual Channel Identity), which can be used as the MID.Without the MID-field, one gains a payload of 10 bits per cell.

AAL5 seems to be the most chosen alternative at manufacturers of ATMequipment and many operators.

If it is supposed that the service quality of the ATM networks is veryhigh, the use of AAL5 saves a lot of bandwidth and proccessing costs forBDS.

AAL5 is simple and easy to work with without too many changes of PDUheaders and end parts.

One disadvantage with AAL5 is that CRC occurs at the end part of CPCSprotocol data unit, as is shown in FIG. 2. If a BDS server in thebroadband network wants to check the validity of the BDS-data which hasbeen transferred by AAL5 units, it must reconfigure the whole CPCSprotocol data unit in the BDS network. The advantage with AAL3/4 wasthat the SAR protocol data units themselves contained check sum fieldsfor the whole SAR protocol data unit. This meant that a BDS server inthe network could check individual cells regarding errors and atdetection of problems reject all following cells belonging to the CPCSprotocol data unit.

In the present invention, the AAL5 CPCS protocol data unit is not at allreconfigured in the overlaying BDS network. Accordingly, in StreamingMode, time and memory demanding processing of AAL5 CPCS protocol dataunits in the servers are avoided. This means that everything in the BDSnetwork is in Streaming Mode even if the BDS users are working inMessage Mode. The underlaying assumption is that the service qualitywhich is supplied by the ATM networks is sufficiently good for leavingmost control functions to higher layers in the end systems. Thissolution should be balanced by an intelligent check of system parameterssuch as the maximum length of BDS data units, size of buffers, and loaddistribution technologies.

As a requirement for the cells being able to reach the BDS server it isassumed that BDS ATM-cells which are coming in on the network are taggedwith VPI (Virtual Path Identity)="BDS". The ATM-backbone (Switches) mustbe so designed that cells with VPI="BDS" are forwarded to the BDSserver.

In FIG. 3 can be seen three different BDS ATM cell types which are to beanalysed by the BDS server. The BOM-cell (Beginning of Message)indicates that the cell is the first in a sequence of cells from a BDSpacket. This cell is the only one in the BDS server which gets itspayload analysed. The analysis aims at identifying a VPI/VCI over whichthe cell is to be routed and is made by translating the E.164-address onsuch a VPI/VCI-pair. Consequently, the invention requires that theE.164-address is available in the payload of the first cell. The rest ofthe cells are either COM (Continuation of Messsage) or EOM (End ofMessage).

Cell types are distinguished based on the bit PT (Payload Type) or AUU(ATM-User-to-User) of the header of the ATM-cell. PT=0 indicates BOM orCOM, whereas PT=1 is for EOM. In order to distinguish between BOM andCOM the server must rely on correct reception of EOM. If an EOM is lost,the server can wrongly take a BOM for a COM.

The implementation of the BDS-code has been performed on top of Foreshost interface, code version 2.1.1. Our intention has been to use asmuch as possible of the existing Fores-code. Consequently, we have usedits segmentation and reconfiguration routines and data structures withsmall modifications for dealing with support of logical channels. Suchsupport is necessary since we tag BDS-cells with VPI="BDS", as havepreviously been mentioned.

Other modifications include the reconfiguration code for supporting thefunction of the BDS-server. The primary contribution has been to supplya thin protocol layer on top of the AAL5-code supplied by Fore, as wellas a Raw contact interface and an IP-interface (Internet Protocol),which allows access to the protocol. The Raw contact interface is mainlyintended for tests and network managing activities.

The terminal code consists essentially of an output routine and an inputroutine which each constructs and analyses the header of a BDS protcoldata unit. The function of the output code is to add a BDS protocol dataunit header to to the protocol of the upper layer. The header containsamong other things source and destination addresses in E.164-format, andan identifier for protocols of the upper layers. The intention is tocreate a new bsd core memory buffer with this information and then addthe user data memory buffer of the upper layer.

The output routine then calls the lower AAL5 protocol. Since this lowerprotocol requires a VPI- and VCI-pair for transmission, a table searchmust be performed which brings back the VPI/VCI-pair which correspondsto a given E.164 destination address.

On the input side, the lower protocol delivers a memory buffer chainwith a BDS protocol data unit header. The header is analysed by theinput routine and is then transmitted to the suitable upper protocoldepending on the protocol identification field in the header. In thepresent embodiment we have chosen to drive this code on the breakinglevel of the hardware, since only a marginal number of instructions mustbe added to the code of the original hardware at the breaking level forperforming the required analysis of the header. The two upper protocolswhich are available are the IP and Raw contact interfaces.

Since the BDS-server works on cell basis, the code must be introduced atthe lowest level in the Fore drive unit. When a cell header has beenread from the input fifo-queue, the code checks if the logical channel10 identity is one which has been appointed to be a part of the BDSnetwork. If yes, a table search shall be performed to find the suitableoutgoing VCI/VPI-pair Posts in the table are maintained on a sessionlevel (by the use of time releasing) rather than on a packet level withregard to the efficeincy.

If the cell is a part of the middle of a BDS packet (i.e., COM) thisimplies stepping of a table. More complicated procedures are required ifthe cell is the first or the last part of a BDS packet. At that, sometable modifications must be performed based on other arguments in thesearch function (a pointer to the cell contents as well as a flag whichindicates if the cell header has an appointed message end bit (EOM)).See also FIG. 3.

The original Fore reconstruction data structure did not support VPI andonly consists of a vector of VCI-posts. Each post registers what type ofadaption layer the VCI is driving and which upper protocol it should besent to. In the case with AAL5, the incoming cells are reconfigured atarrival until a whole AAL5-message has been delivered. At this point oftime the whole AAL5-packet is sent to the next protocol layer.

In order to support the logical channel identifier VPI we have modifiedthe reconfiguration data structure to a matrix, where each post isindexed by a VCI/VPI-pair. All VCI which are a part of BDS VPI aremarked as BDS posts. End-BDS-cells will by that be identified andcollected in their respective posts. When a whole AAL5-packet has beenreconfigured again in a BDS-post, the packet is sent to the BDS inputfunction.

A Raw contact interface was chosen to give direct access to the BDSprotocol in the present embodiment. The main reason for using the Rawcontact interface was that many code and data structures are obtainedinto the bargain, especially protocol control block managing routines(PCB Protocol Block) and data structures. The Raw contact interfaceworks with the generic contact address structure for describing sourceand destination addresses. Since the system works with a completely newaddress family based on E.164-addresses (ISDN-numbers), a new contactaddress was created which might be formed on the generic contact addressstructure. A protocol switch for this address family was also added.

The only protocol which is available in this protocol switch is the BDSprotocol. The contact flags have been defined so the messages are sentseparately and the protocol represents the source address with eachmessage. All protocol control block managing is performed with alreadyexisting Unix bsd-code and the generic routine can be used withoutmodifications.

The output routine quite simply forms the generic source and destinationcontact addresses which are stored in the Raw protocol control block ofcontact-address-BDS-structures and then calls a BDS output function withsuitable arguments.

On the input side, the BDS function assumes that all protocolidentifiers in the BDS protocol data unit header which are not IP areintended for the Raw contact interface. Packets arranged for thisinterface consequently get their headers removed and the generic Rawinput routine is called which fit source and destination addresses withcorresponding protocol control blocks and contact identifiers.

In FIG. 4 is shown a sketch over software modules for the BDSimplementation. To introduce a new Internet-protocol interface (IP)implies adding attach routines for drive units to this interface.Because the Fore drive unit supports IP directly over the adaptionlayer, a second IP interface must be created for IP over BDS. Inpractice an interface network structure is defined and attached by useof an interface attachment routine. The interface network structuresdefine interface by specifying name, address, interface characterisics,routines and statistics of the interfaces. This interface networkstructure is very like the one which is defined for IP direct over theadaption level and the differences are that another IP-address as wellas a new output routine are used.

The output routine must make a table search for converting thedestination IP-address to a corresponding E.164-address. This tablecorresponds to the ARP tables search which must be done for IP overEthernet. It will then call the BDS output routine with protocolidentifier argument set to IP.

On the input side, the BDS input function recognizes IP-packets by theprotocol identifier field of the protocol data unit being set to IP.These packets are then put in the IP input queue and a software break isposted.

The present embodiment is based on the assumption that the underlyingATM network supplies a sufficiently good service quality, so the servercan assume that ATM-cells which are produced from BDS terminals areessentially free from errors.

The main advantage with an overlaying BDS network is how easy it is tointroduce direct BDS services in ATM. This, however, implies that a port(in/out) to the switch to which the server is connected must be reservedfor this marginal service. A physically overlaying network also causes aduplication of the BDS traffic volume from the ATM switches point ofview. Ideally the BDS server should be implemented in the switch itself;in fact the BDS-server is a Streaming Mode in the switch with exceptionfor the first cell in which it must regard the payload. A reallyoverlaying BDS network would furthermore probably never work completelyin Streaming Mode, because the BDS server and the terminals would findit too expensive expressed in processing costs to enforce breaks on cellbasis.

Accordingly a device has been described which solves the, by way ofintroduction, mentioned problems.

What is claimed is:
 1. A telecommunication system comprising:a LocalArea Network (LAN); an Asynchronous Transmission Mode (ATM) network; anda Broadband Data Service (BDS) network connecting the LAN and the ATMnetwork and including at least one BDS server configured to perform aconnectionless service function to transmit cells between the LAN andthe ATM network without reconfiguring a protocol data unit of a cell inthe BDS network, wherein transmission of information in the BDS networkis in streaming mode while BDS users work in a message mode.
 2. Thesystem according to claim 1, wherein the at least one BDS serverincludes a plurality of BDS servers and each of the plurality of BDSservers decides a next node in which a cell must be transmitted to reachits final destination.
 3. The system according to claim 1, wherein theATM network includes at least one switch which forwards BDS ATM-cellshaving a Virtual Path Identity (VPI) field of "BDS" to the at least oneBDS server.
 4. The system according to claim 1, wherein the ATM networkis configured to use an ATM Adaption Layer (AAL5) protocol.
 5. Thesystem according to claim 1, wherein the at least one BDS serveranalyzes a BDS ATM cell transmitted from the ATM network, and determinesif the BDS ATM cell is one of 1) a Beginning Of Message (BOM) cell, 2) aContinuation Of Message (COM) cell, or 3) a End Of Message (EOM) cell.6. The method according to claim 5, wherein the at least one BDS serveridentifies a VPI/VCI over which the BDS ATM cell is to be transmittedwhen the at least one BDS server determines the BDS ATM cell is a BOMcell, and transmits the BDS ATM cell to its final destination over theidentified VPI/VCI.
 7. A telecommunication method, comprising the stepsof:connecting a Local Area Network (LAN) to an Asynchronous TransmissionMode (ATM) network via a Broadband Data Service (BDS) network includingat least one BDS server; and transmitting cells between the LAN and theATM network without reconfiguring a protocol data unit of a cell in theBDS network, wherein transmission of information in the BDS network isin streaming mode while BDS users work in a message mode.
 8. The methodaccording to claim 7, further comprising the step of:deciding, by the atleast one BDS server, a next node in which a cell must be transmitted toreach its final destination.
 9. The method according to claim 7, furthercomprising the step of:forwarding, by the ATM network, BDS ATM-cellshaving a Virtual Path Identity (VPI) field of "BDS" to the at least oneBDS server.
 10. The method according to claim 7, further comprising thestep of:configuring the ATM network to use an ATM Adaption Layer (AAL5)protocol.
 11. The method according to claim 7, further comprising thesteps of:analyzing, by the at least one BDS server, a BDS ATM celltransmitted from the ATM network; and determining, by the at least oneBDS server, if the BDS ATM cell is one of 1) a Beginning Of Message(BOM) cell, 2) a Continuation Of Message (COM) cell, or 3) a End OfMessage (EOM) cell.
 12. The method according to claim 11, furthercomprising the steps of:identifying, by the at least one BDS server, aVPI/VCI over which the BDS ATM cell is to be transmitted when thedetermining step determines the BDS ATM cell is a BOM cell; andtransmitting, by the at least one BDS server, the BDS ATM cell over theVPI/VCI identified in the identifying step.
 13. A telecommunicationsystem, comprising:means for connecting a Local Area Network (LAN) to anAsynchronous Transmission Mode (ATM) network via a Broadband DataService (BDS) network including at least one BDS server; and means fortransmitting cells between the LAN and the ATM network withoutreconfiguring a protocol data unit of a cell in the BDS network, whereintransmission of information in the BDS network is in streaming modewhile BDS users work in a message mode.
 14. The system according toclaim 13, further comprising:means for deciding, by the at least one BDSserver, a next node in which a cell must be transmitted to reach itsfinal destination.
 15. The system according to claim 13, furthercomprising:means for forwarding, by the ATM network, BDS ATM-cellshaving a Virtual Path Identity (VPI) field of "BDS" to the at least oneBDS server.
 16. The system according to claim 13, furthercomprising:means for configuring the ATM network to use an ATM AdaptionLayer (AAL5) protocol.
 17. The system according to claim 13, furthercomprising the steps of:means for analyzing, by the at least one BDSserver, a BDS ATM cell transmitted from the ATM network; and means fordetermining, by the at least one BDS server, if the analyzed BDS ATMcell is one of 1) a Beginning Of Message (BOM) cell, 2) a ContinuationOf Message (COM) cell, or 3) a End Of Message (EOM) cell.
 18. The systemaccording to claim 17, further comprising the steps of:means foridentifying, by the at least one BDS server, a VPI/VCI over which theBDS ATM cell is to be transmitted when the determining means determinesthe BDS ATM cell is a BOM cell; and means for transmitting, by the atleast one BDS server, the BDS ATM cell over the VPI/VCI identified bythe identifying means.